Pipes of polymer material are frequently used for various purposes, such as fluid transport, i.e. transport of gases or liquids. The fluid may be pressurised such as when transporting natural gas or tap water, or not pressurised such as when transporting sewage (wastewater), drainage (land and road drainage), for storm water applications or for indoor soil and waste. Moreover, the transported fluid may have varying temperature, usually within the temperature range from about 0° C. to about 50° C. Pressureless (not pressurised) pipes may also be used for cable and pipe protection.
Such pressureless pipes are herein also referred to interchangeably as sewage pipes and pressureless sewage pipes.
The term “pipe” as used herein is meant to comprise pipes in a broader sense, as well as supplementary parts like fittings, valves, chambers and all parts which are commonly necessary for e.g. a sewage piping system.
Pipes according to the present invention comprise single or multilayer pipes, where for example one or more of the layers is a metal layer and which may include an adhesive layer. Structural-wall pipes, such as corrugated pipes, double-wall pipes with or without hollow sections, are also comprised by the present invention.
Different requirements are imposed on pipes for the transport of pressurised fluids (so-called pressure pipes) and pipes for the transport of non-pressurised fluids such as sewage (so-called pressureless pipes). While pressure pipes must be able to withstand an internal positive pressure, i.e. a pressure inside the pipe that is higher than the pressure outside the pipe, pressureless pipes do not have to withstand any internal positive pressure, but are instead required to withstand an external positive pressure, i.e. the pressure outside the pipe is higher than the pressure inside the pipe. This higher outside pressure may be due to the earth load on the pipe when submerged in the soil, the groundwater pressure, traffic load, or clamping forces in indoor applications. There is thus a clear distinction between pressure pipes on the one hand and pressureless pipes on the other hand. As mentioned above, the present invention relates to pressureless pipes.
Pressureless pipes such as sewage pipes are made in a variety of dimensions from about 0.1 to about 3 m diameter and of a variety of materials such as ceramics (vitrified clay mainly), concrete, polyvinyl chloride (PVC), and polyethylene (PE). While ceramics and concrete are low-cost materials, they are unfortunately heavy and brittle. There has therefore been a trend during recent years to replace sewage pipes of ceramics or concrete with pipes of polymer materials such as PVC or PE. While PVC costs less than PE per unit weight, PE has advantages over PVC in other respects by having a lower density and thus a lower weight per meter pipe, having superior low temperature properties, and being weldable. Up to now unimodal polyethylene has been used for making sewage pipes of PE.
Sewage pipes of PE must fulfil at least two fundamental criteria. Firstly, they must show sufficient stiffness to withstand the earth load without any help from internal pressure. The stiffness of the pipe is derived primarily from the pipe material and as a measure of the stiffness may be taken the elasticity modulus (or modulus for short) of the pipe material. The higher the modulus of the pipe material, the stiffer the pipe will be. The stiffness of the pipe may be further enhanced by the design of the pipe wall, e.g. by corrugating the pipe. Secondly, the pipe should not be brittle, because if it is too brittle the pipe will fail due to brittle cracking. A measure of the brittleness of the pipe is its resistance to slow crack growth. The higher the resistance to slow crack growth, the better.
When a material with a higher modulus is used a thinner pipe wall may be used while obtaining the same or higher (ring) stiffness as a lower modulus pipe with a thicker pipe wall.
Thinner pipe walls are more sensitive to cracks since any damage or notch on the pipe surface will propagate easier through the pipe wall. Structured-wall pipes (corrugated, ribbed, twin-wall pipes, etc.) are most sensitive to cracks and the slow crack growth properties of the material since the structured pipe design often consists of thin sections.
Structured-wall pipes includes for example single-layer corrugated pipes, ribbed pipes, twin-wall pipes with hollow sections, multilayer pipes with or without hollow sections or foamed layers, and spirally wound pipes with or without hollow sections with smooth or corrugated pipe design.
Basically, pipes with thin sections, either smooth solid-wall pipes of smaller diameters or structured-wall pipes with thin sections are more sensitive to cracks. Due to the high and 3-dimensional structure of structured-wall pipes also the stresses are locally higher when subject to external load conditions compared to smooth solid-wall pipes, i.e. larger sensitivity to cracks.
The stiffness and brittleness are two contradictory properties. Accordingly, the stiffer a pipe is, the more brittle it will normally be. Thus, a high modulus is usually accompanied by a low resistance to slow crack growth. As an illustration, the modulus of the unimodal PE materials hitherto used for pressureless pipes has been limited to about 1100 MPa, because at this level the slow crack growth is only 1.5 hrs, measured according to ASTM F 1473 (2.8 MPa/80° C.).
In view hereof there is a need for an improved pressureless pipe of a polymer material that combines a high stiffness and a low brittleness, i.e. has a high modulus and a high resistance to slow crack growth.
Further, pressureless pipes are often exposed to high as well as low temperatures. They must therefore be durable within a wide range of temperatures which means that they should display a high impact strength, particularly at low temperatures.
In this connection it should be mentioned that through EP 1 192 216 (which corresponds to WO 01/02480) a polyethylene moulding material is known which is used for gas and water pipes. This polyethylene moulding material has a bimodal mass distribution, a total density of at most 0.948 g/cm3, and a melt flow index, MFI190/5 of at most 0.2 dg/min. The polyethylene moulding material comprises 35-65% by weight of a low molecular ethylene homopolymer with a viscosity number VZA of 40-90 cm3/g, a melt flow index MFI190/2.16 of 40-2000 dg/min, and a density of at most 0.965 g/cm3; and 35-65% by weight of a high molecular ethylene copolymer with a viscosity number VZB of 500-2000 cm3/g, a melt flow index MFI190/5 of 0.02-0.2, and a density of 0.922-0.944 g/cm3. The high molecular ethylene copolymer preferably contains 2.5-4% by weight of a comonomer with 4-10 carbon atoms. Furthermore, the fraction of the polyethylene moulding material recovered in a TREF analysis at 78° C.±3 K has a mean molar mass of at most 200 000 g/mol. It should be pointed out that this reference relates to pressure pipes in contrast to the present invention which relates to pressureless pipes.
Other documents describing the technical background which might be of interest are EP 1 146 079, WO 00/01765, WO 00/22040 and WO 03/102075. However, all these references relate to pressure pipes in contrast to the present invention which relates to pressureless pipes.